1. Field of the Invention
The present invention relates to a semiconductor laser device, and more particularly to a semiconductor laser device used as a light source for optical information processing, a signal source for optical communications, or an excitation light source for fiber amplifiers.
2. Background Art
FIG. 20 is a diagram of a fiber grating laser taken from the journal article xe2x80x9cFiber Grating and Its Applicationsxe2x80x9d by Tohru Inoue, in IECE transactions, Vol. 85, No. 2, pp 126-128, February, 2002. In a fiber grating laser 52 shown in the figure, a fiber grating 54 having a reflectance of Rfg is provided within an optical fiber 53. Light of a specific wavelength entering the optical fiber 53 is reflected by the fiber grating 54. On the other hand, a semiconductor laser 55 is configured such that its front end face 56 has a reflectance (Rf) corresponding to low reflection or no reflection, and its back end face 57 has a reflectance (Rr) corresponding to high reflection. Reference numeral 58 denotes the optical waveguide region of the semiconductor laser 55. In this arrangement, a resonator is formed between the fiber grating 54 and the back end face 57 of the semiconductor laser 55, thereby stabilizing the oscillation wavelength of the laser light.
Use of such a fiber grating laser, however, makes the structure of the device complicated. Furthermore, it is necessary to combine the semiconductor laser and the optical fiber, producing a coupling loss therebetween.
The present invention has been devised in view of the above problems. It is, therefore, an object of the present invention to provide a semiconductor laser device capable of stably extracting from its semiconductor laser element laser light whose wavelength exhibits only a small change, without using any fiber grating.
Other objects and advantages of the present invention will become apparent from the following description.
JP-A No. 10-186104 discloses a multilayer antireflective film whose reflectance curve has two separate bottoms (low reflectance points) and a peak therebetween in a required wavelength band. Further, JP-A No. 11-214799 discloses a multilayer film whose reflectance is 10xe2x88x924% to 10% at the center wavelength and the reflection spectrum characteristic curve has minimum values on both sides of the center wavelength. Still further, JP-A No. 2-241075 discloses a semiconductor laser device in which a film whose reflectance depends on the wavelength is formed.
However, the multilayer antireflective film described in the JP-A No. 10-186104 was devised to obtain a low reflectance over a wide range of wavelengths, and therefore the oscillation of the semiconductor laser is suppressed at each of the two bottoms and the peak. The purpose of the present invention, on the other hand, is to stably extract from a semiconductor laser element itself laser light whose wavelength exhibits only a small change, without using any fiber grating. Furthermore, as described later, the multilayer films of the present invention are used to cause the semiconductor laser to oscillate at a wavelength xcex1 at which the reflectance is maximized. Therefore, the purpose and the effect of the present invention are different from those described in the above patent publication.
Further, the refractive indexes of first films for realizing the reflectance characteristics described in the above three patent publications are each larger than the square root of the effective refractive index nc of a respective semiconductor laser element. Therefore, these multilayer films are different from those of the present invention.
The features and advantages of the present invention may be summarized as follows.
According to one aspect, a semiconductor laser device comprises a semiconductor laser element having an effective refractive index of nc, and a multilayer film formed on at least one of the two end faces of the semiconductor laser element. The multilayer film has a structure in which a first film with a refractive index of n1 and a second film with a refractive index of n2 are alternately laminated, the first film being formed such that the first film is in contact with the one of the two end faces. The refractive index n, and the refractive index n2 satisfy relations expressed by the formulas, n1 less than (nc)1/2 and n2 greater than (nc)1/2. A reflectivity characteristic of the multilayer film is such that a reflectivity of the multilayer film is maximized at a wavelength xcex1 in a predetermined wavelength region and minimized at wavelengths xcex2 and xcex3 on a shorter-wavelength side and a longer-wavelength side of the wavelength xcex1, respectively.
Other and further objects, features and advantages of the invention will appear more fully from the following description.